Lasers can be used to locally heat a test structure. Localized heating produces a temperature gradient, which, in turn, produces stress and strain in the heated area. If the heating varies with time, then the resultant stress and strain will also vary with time. Time varying stress and strain produce acoustic/sound waves.
Pulsed lasers have been utilized to produce a thermal impulse response in a test structure. The resultant acoustic pulse is then detected after propagation through the test structure. These techniques are utilized to determine the acoustic propagation properties of the test structure. The acoustic propagation properties can be utilized to identify subsurface flaws and defects. These techniques are equivalent to standard ultrasound techniques utilized for the same purpose. A uniform surface structure is desirable for these techniques in order to obtain a constant amplitude and phase of the laser generated acoustic pulse.
A continuous wave (CW) laser impinging onto a test structure will produce no acoustic waves (sound). Scanning of a CW laser will produce time dependent heating. However, a uniform surface structure is still unlikely to produce any acoustic waves. It has been determined experimentally that scanning of a CW laser over a non-uniform test structure will produce acoustic waves. In particular, test structures with multiple materials present, e.g. a printed circuit board with components soldered into place, will produce a series of “pops and pings”, i.e. a laser thermal-acoustic signal. These pops and pings are a result of the differential expansion and contraction causing a release of built up stress and strain of the differing materials as they are heated and cooled. In particular, components, e.g. vias, solder joints, etc., on the test structure will produce different acoustic signatures, depending on the quality of that component.
Therefore, there exists a need to identify thermal response to determine the quality of the test structure components.